Internet protocol multimedia subsystem (IMS) is a known architecture, which unites public communication networks including a fixed telephone network and a mobile communication network based on Session initiation protocol (SIP) and delivers multimedia services. The IMS architecture is standardized by 3rd Generation Partnership Project (3GPP), which is a standards organization for third-generation mobile phone standards. Part of the IMS architecture is already employed in a third-generation mobile communication system that is in conformity with the International mobile telecommunications (IMT)-2000 standard and in a 3.9-generation mobile communication system that follows the third-generation mobile communication system. The IMS architecture is already used to offer communication services including Code division multiple access (CDMA), which is compatible with the third-generation mobile communication system, and Long term evolution (LTE), which is compatible with the 3.9-generation mobile communication system.
In a mobile communication network, a plurality of exchanges that process a call control signal for controlling the communication between a caller terminal and a callee terminal are accommodated. When the plurality of exchanges sequentially process the call control signal, the communication connection between the caller terminal and the callee terminal is established. Examples of the caller terminal and the callee terminal include a mobile terminal, such as a mobile phone or a portable information terminal.
In case of a large-scale disaster, it may be difficult to establish the communication connection between the caller terminal and the callee terminal. One of possible causes is that so many calls are originated for safety confirmation and so many call control signals are generated that the exchange that has received the call control signals may lose its processing capability. In this case, a measure to restrict the process for the call control signal is taken in the exchange so as to avoid a system crash that may be caused by the congestion state continuing in the exchange. For example, audio guidance is provided to the caller terminal that has failed to establish communication connection based on the restriction measures taken when the exchange enters the congestion state so that the caller terminal is informed that the call originated by the caller terminal is unable to be connected because of the heavy communication. Typically, when no communication is established, a caller terminal operation causes the caller terminal to stop the call for a time and originate a call again after that to attempt to establish communication connection.
There is a known technique, which avoids such a call reoriginating operation of the caller terminal in the above-described case. According to the technique, the acceptance time at which a call is received and the like are registered in a registration list of an exchange on the callee side in response to a request from the callee terminal, a number and the like of the caller terminal that satisfies conditions are registered in an acceptance list, the communication with the caller terminal is caused to end, and the communication connection is performed in order of the registration in the acceptance list after the end of the acceptance (see, for example, Japanese Laid-open Patent Publication No. 8-195748). The technique allows communication connection to be established after the congestion state is released, and unless the congestion state is released, no communication connection between the caller terminal and the callee terminal may be established.